As the size of the transistor continues to scale down beyond 193i lithography resolution, the need to engineer new alternatives for patterning which enable smaller critical dimensions (CD) arises. One such technique is the so called triple Litho-Etch (usually abbreviated to LELELE, and sometimes also written as 3×LE or LE3×).
This technique involves dividing the neighboring structures (blocks) into 3 separate lithography masks. U.S. Patent Application Pub. No. 2014/0237436, for example, proposes a layout decomposition for triple patterning lithography. Each structure is printed via lithography exposure, followed by a dry (plasma) etch step which transfers this structure to the core hard mask below the photo-resist stack layers. During this transfer of pattern, the dimension of this pattern needs to be scaled down to the actual critical dimension required. Currently, this scaling down can be done by two different methods.
Two current solutions to shrink the critical dimension of negative tone lithography 3×LE are as follows (applied on an etch stack comprising a target layer, covered by a hard mask, covered by an amorphous carbon layer, covered by a bottom antireflective coating, covered by a photoresist layer):
One solution is by shrinking the litho gap in the photoresist layer by depositing organic layers over the resist after resist development. This way, as the organic layers deposit on the side walls, the critical dimension of the gap reduces as required.
Another solution is where the litho gap is transferred into the bottom antireflective coating. Then, as this gap/pattern is transferred into the amorphous carbon layer, a highly controlled etch process is executed, which tapers the side walls of the core layer, reducing the critical dimension at the bottom of the gap/feature in the core layer.
Both of these techniques need three times the amorphous carbon layers to be deposited and etched to transfer the three lithography patterns into the final hard mask which can be an a-Si layer (above the target layer).
Additionally, both of these techniques need further etch-deposition of hard-mask layers and processing to reverse the gap to a block shape. This is because at the end, blocks in the shape of these gaps are required to be transferred into the dense structures previously created by a multi-patterning process such as SAQP or DSA.
There is still room for improvement.